Power Requirements and Noise Emission of Circular Saw During the Cutting of Solid Wood

Original paper

THE INFLUENCE OF WOOD MOISTURE CONTENT ON THE PROCESS OF CIRCULAR RIP-SAWING. PART I: POWER REQUIREMENTS AND SPECIFIC CUTTING FORCES

Authors

Ružica Beljo-Lučić, Vlado Goglia, Stjepan Pervan, Igor Đukić, Stjepan Risović

Authors are assistant professor, professor, assistant, assistant and assistant professor at Faculty of Forestry the University of Zagreb

Address:

Faculty of Forestry

P.O. Box 422

10002 ZAGREB

C R O A T I A

Corresponding author:

Assist. Prof. Ružica Beljo-Lučić, PhD

Fax number: ++385-1-218-616

Telephone number: ++385-1-235-2430

E-mail:

Titles of authors:

Assist. Prof. Ružica Beljo-Lučić

Prof. Vlado Goglia, PhD

Assist. Stjepan Pervan, PhD

Assist. Igor Đukić, BSc

Assist. Prof. Stjepan Risović, PhD
ABSTRACT

This paper presents the results of research on power requirements during conventional sawing of oakwood (Quercus Robur L.) on a circular saw for rip-sawing. The major parameters affecting the cutting power are the basic density and the nature of wood species, wood moisture content, feed direction, feed rate, cutting depth and kerf width. The concerned variables in this research were wood moisture content, cutting depth and feed rate. The cutting speed and tool parameters remain constant. Directions of cutting with regard to grain directions were approximately the same for each cutting depth.

Cutting power was measured for a specific condition of wood and machine parameters and specific cutting forces were estimated. Specific cutting forces during sawing of green wood are less than specific cutting forces of kiln dried wood and frozen wood. Research results indicate a significant decrease of specific cutting forces in sawing green wood in transversal direction (90º-90º).

Key words: circular saw, wood moisture content, power requirements, specific cutting forces

INTRODUCTION

For achieving efficient wood machining with circular saws it is essential to understand thoroughly the interaction between raw materials, end products, machinery and the sawing process itself. Adequate knowledge of correct bite, feed speeds, tooth speeds, side clearances, depths of cut, cutting forces and power requirements is essential for a good saw operation.

For pulling out a chip the teeth of a circular saw must apply a force bigger then the cohesion of the material. This effort, called cutting force, is the result of several forces: bending of fibres, shearing force and friction on tool faces (Aquilera and Martin, 2001).

The cutting force as well as power requirements are dependent on wood species (wood density, nature of the species and moisture content), work conditions (feed direction, depth of cut, kerf width, chip size, feed and sawing speed) and tool parameters (hook angle, clearance angle, edge sharpening, total number of teeth and tool diameter). We have a good understanding of the influence of most wood and machinery parameters on cutting forces and power requirements (Postnikov, 1965; Mikolašik, 1981; Lunstrum, 1985; Williston, 1989; Goglia, 1994).

Aquilera and Martin (2001) pointed out the following variables as the ones having the highest impact on cutting forces: depth of cut, wood density and feed direction. Konishi (1972) stated that cutting force is affected quite sensitively by wear of edge. He stated also that the more specific gravity was, the more the cutting force, and this trend is evident in the case of the same species. It makes sense since the nature and origin of species could be the factors having significant impact on cutting forces.

The consumed cutting energy is directly linked to the forces acting on the tool. The cutting power Pc can be expressed as follows:

(1)

where Vs is the wood stock removed in cubic centimetres per second and Ws is specific energy required for transforming the cubic centimetre of removed stock. This specific energy corresponds to the value of specific cutting force. So, the cutting power corresponds to wood volume Vs transformed to chips per second multiplied by the specific cutting force fs.

(2)

where vf is feed speed, hc is depth of cut and sk is kerf width.

According to some authors the specific cutting force is exclusively a function of parameters associated with the relevant machining properties of the material (Bučar and Bučar, 2002; Aquilera and Martin, 2001) and others state that the specific cutting force is also influenced by technical parameters of cutting (Williston, 1989; Goglia at al, 2002). It is obvious that the determination of the function of influencing parameters on specific cutting force is very complex.

Krilov (1980) developed the mathematical expression that covered influence of basic wood characteristics on sawblade potential. He stated that one of the basic wood characteristics greatly affecting the behaviour of the cutting tool was undoubtedly timber moisture content but his empirical equation was valid for the constant moisture content.

Most researches of parameters influencing cutting forces and power requirements were conducted at the constant moisture content of wood (Konishi, 1972; Steward, 1984; Aquilera and Martin, 2001; Bučar and Bučar, 2002).

As reported by Postnikov (1965) and Mikolašik (1981) the specific cutting force in sawing as well as power requirements increase with the increase of wood moisture content in spite of the decrease of wood mechanical characteristics. They explained that the increase of forces of friction between the tool, pulled out chips and both workpiece kerf sides occurred more rapidly with the increase of wood moisture than the decrease of wood mechanical characteristics. Also, Ratnasingam at al. (1999) in determining the costs of wood machining process took into account the power increase when machining wood with a higher moisture content.

The purpose of our investigation was to measure power requirements of the circular saw in cutting green and kiln dried wood. The secondary objective was to determine specific cutting forces in cutting green and dried wood pieces and to make comparison between them.

MATERIAL AND METHODS

This study researches cutting characteristics of the most important and most valuable wood species in Croatia: the oak wood (Quercus Robur L.). The machining process was carried out on a circular saw for rip-sawing “Bratstvo”, type AC-3. The cutting speed was 53 m/s. Tool parameters are given in the Table 1.

Tab. 1: Characteristics of used circular saw

Cutting tests were performed on green wood (moisture content was 40-70 %) and kiln dried wood pieces (moisture content was 8-12 %), on three cutting depths (24 mm, 40 mm and 52 mm) and four feed speeds (15, 23, 30 and 46 m/min) keeping cutting speed constant. The measurement was also performed on a wood piece (cutting depth 52 mm) that was frozen. The directions of cutting with regard to grain directions in the whole experiment were approximately the same for each cutting depth (see Fig. 1). The moisture content was determined by use of electrical resistant wood moisturemeter according to EN 175-13.01-2.

Fig. 1: Sawing condition

In the sawing process it is very important to measure cutting forces and the specific cutting force for determining the working conditions but in practice this proves to be difficult. It is easier to measure the cutting power. Aquilera and Martin (2001) showed that the cutting force could be estimated by measuring the cutting power for a specific wood and machine condition.

The electrical power requirements during cutting were measured by use of the instruments shown in Figure 2.

Fig. 2: The arrangement of used measuring equipment (1 – sawblade, 2 – electromotor, 3 – current transformer, 4 – programmable power converter Iskra MI400, 5 – National Instruments BNC-2110, 6 – NI DaqPad 6070-E, 7 – personal computer)

Cutting power Pc was derived according to the next relation:

(3)

where: Pt – arithmetic mean of measured total power in 0.2 s interval (the sampling frequency was 80 kHz); Po - arithmetic mean of measured power during machine idling; PLel – loss of electric power because of motor loading; PLm – loss of mechanic power because of motor loading.

The data were analysed by use of: LabVIEW, EXCEL and STATISTICA 6.0.

RESULTS AND DISCUSSION

Figure 3 shows the results of cutting power in relation to feed speed for green, frozen and dried wood pieces and for three different cutting depths. In the range of the experiment feed speed it is obvious that the dependence of the cutting power on feed speed could be expressed as linear.

Fig. 3: Cutting power in relation to feed speed for green, frozen and dried wood

For all cutting depths, measurement results have shown that more power is required when sawing dried and frozen wood than green wood. This result is contrary to the assumption of the above authors that the power requirements increase with the increase of wood moisture content. Also, the statement of the majority of authors researching cutting forces and power requirements in cutting wood, that the power requirements increase with the increase of wood density could be valid only for the totally dry wood because higher moisture content increases the wood density but not power requirements.

The specific cutting forces were determined using the relation:

(4)

where Pc is cutting power, vf is feed speed, hc is depth of cut and sk is kerf width.

Figure 4 shows the calculated specific cutting forces for dried, green and frozen wood in the range of the experiment feed speeds. The changes of specific cutting force influenced by different feed speed in the range of experiment are not significant. The reason lies in relatively high feed rates and average chip thickness exceeding 0.15 mm.

Fig. 4: Calculated specific cutting forces in relation to feed speed for green, frozen and dried wood

Figure 4 shows clearly that specific cutting forces of dried wood are greater than specific cutting forces of green wood. It seems that the effect of moisture content in decreasing wood mechanic characteristics has higher influence on the specific cutting forces than the effect of wood moisture in increasing friction between the tool, pulled out chips and both workpiece kerf sides. We conducted statistical testing to support our hypothesis. As shown in Figure 5, the variance of specific cutting forces of dried wood is greater than the variance of specific cutting forces of green wood.

Fig. 5: Median values, minimum and maximum values of specific cutting forces for green, dried and frozen wood

As data variances were not homogenous, nonparametric testing was used for statistical analyses, namely the Mann-Whitney U-test. The statistical test confirmed our hypothesis and the differences between specific cutting forces of green and dried wood are significant at p<0,01. Also, the specific cutting forces of frozen wood are greater than specific cutting forces of green wood if the other parameters are the same.

The specific cutting force of dried wood is greater for the cutting depth of 24 mm than for the cutting depth of 40 mm and 52 mm. This is the consequence of practically transversal direction of cutting (90º-90º) when the cutting depth of circular sawing is small. When the cutting depth is great the angle between cutting direction and the grain direction is less then 90º and the specific cutting force is smaller. It is interesting that the difference of specific cutting forces between different cutting depths, that means different cutting directions, for green wood is not significant at the level p<0,01. It could be explained by a significant decrease of specific cutting forces in transversal cutting direction caused by decreased wood mechanical properties as a result of high wood moisture content.

The conducted experimental sawing clearly shows that less energy is required for sawing green wood than dried and frozen wood. It should be noted, however, that further research should be carried out of other machining parameters, sawing quality parameters and other wood species so as to make a comprehensive comparison between sawing of dried and green lumber.

CONCLUSION

Contrary to the opinion of some authors, the research results have shown that the specific cutting force in sawing green wood is lower than the specific cutting force in sawing kiln dried wood. It seems that the effect of moisture in decreasing wood mechanic characteristics has a higher influence on the specific cutting force than the effect of wood moisture in increasing friction between the tool, pulled out chips and both workpiece kerf sides. Research results indicate a significant decrease of specific cutting forces of green wood especially in transversal direction of cutting. According to conducted research, rip-sawing of green lumber requires 20 – 30 % less power than sawing of kiln dried wood and this fact should be taken into account of wood machining costs.

REFERENCES

1. Aquilera, A, Martin, P. 2001: Machining qualification of solid wood of Fagus silvatica L. and Picea excelsa L.: cutting forces, power requirements and surface roughness. Holz als Roh- und Werkstoff 59:483-488.
2. Bučar, B., Bučar, D. G., 2002: The influence of the specific cutting force and cross-sectional geometry of a chip on the cutting force in the process of circular rip-sawing. Holz als Roh- und Werkstoff 60:146-151.
3. Goglia, V., 1994: Strojevi i alati za obradu drva I dio, Šumarski fakultet Zagreb, p.73-74.
4. Goglia, V., Risović, S., Beljo-Lučić, R., Đukić, I., 2002: Mehanika kružnih pila, I. dio: Izbor položaja lista pile. Drvna industrija 53(2):93-98.
5. Krilov, A., 1980: Toward the re-appraisal of the influence of basic wood characteristics on sawblade potential. Holz als Roh- und Werkstoff 38:145-149.
6. Konishi, C., 1972: Cutting ability of knife planer. I. Relationship between condition of wood and cutting force. Mokuzai Gakkaishi 40(6): 577-583.
7. Lunstrum, S. J., 1985: Balanced Saw Performance. Technical Report No. 12, Madison, WI. USA, p. 17
8. Mikolašik, L., 1981: Drevarske stroje a zariadenia, 1. zvazok, SNTL – Statni nakladatelstvi technicke literatury, Praha, p. 38
9. Postnikov, A., 1965: Mašine za mehaničku obradu drva I dio, teorija rezanja. Univerzitet u Sarajevu, p. 60-61.
10. Steward, H. A., 1984: Result force and surface quality from some face-milling variables. Forest Product Journal 34(5): 21-24.
11. Ratnasingam, J., Ma, T. P., Perkins, M. C., 1999: Productivity in wood machining processes – a question of simple economics. Holz als Roh- und Werkstoff 57:51-56.
12. Williston, E.D.M., 1989: Saws. Design, Selection, Operation, Maintenance. Second Edition. San Francisco, California: Miller Freeman Publications. p. 450.

Tab. 1: Characteristics of used circular saw

Properties

Value

Fig. 1: Sawing condition

Fig. 2: The arrangement of used measuring equipment (1 – sawblade, 2 – electromotor, 3 – current transformer, 4 – programmable power converter Iskra MI400, 5 – National Instruments BNC-2110, 6 – NI DaqPad 6070-E, 7 – personal computer)

Fig. 3: Cutting power in relation to feed speed for green, frozen and dried wood

Fig. 4: Calculated specific cutting forces in relation to feed speed for green, frozen and dried wood

Fig. 5: Median values, minimum and maximum values of specific cutting forces for green, dried and frozen wood